Thermodynamic performance optimization of two-stage hydraulic-driven piston hydrogen compressors based on variable speed control method

The two-stage hydraulic driven piston compressor is increasingly used in 70 MPa hydrogen refueling stations. Due to variable suction pressures as hydrogen is drawn from trailer into the compressor, unbalanced distribution of pressure ratios, and hereby power consumption and discharge temperatures may occur between two stages, consequently resulting in excessively large power and high discharge temperature for a single stage. This paper proposes a scheme of equalizing pressure ratios of the two stages at any varied suction pressures by regulating the oil supply rate of the hydraulic cylinder of each stage separately. Thermodynamic simulations at varied suction pressures were conducted for the compressor with constant speed and variable speed, respectively, and a test rig was built up to validate the simulated results. The results indicate that almost identical pressure ratios between two stages were achieved by regulating the speed of each stage. As a result, the difference in power between the two stages was reduced from 35.8 kW to 11.9 kW, while the temperature deviation between the two stages was decreased from 200.8 K to 5.6 K. Furthermore, the highest discharge temperature of the two stages compressors dropped from 603.9 K to 501.1 K.